1. Field of the Invention
The present invention relates to a printed circuit board and, more particularly, to a method for interconnecting a multi-layer printed circuit board capable of electrically connecting circuit patterns stacked by multiple layers.
2. Background of the Related Art
FIG. 1 illustrates a conventional printed circuit board fabrication method that is disclosed in Korean Patent No. 203,540 (corresponding U.S. Pat. No. 5,600,103). As shown in FIG. 1, a process in which a conductive paste is coated on a copper foil pattern 3 by a screen printing method is repeatedly performed to obtain a conical conductor bump 2′ grown to a predetermined height. Then, a synthetic resin-based film type support body 4 is positioned on the conical conductor bump 2′ and pressed with a roller or the like, so that the conical conductor bump 2′ can penetrate and protrude from the synthetic resin-based support body 4.
Thereafter, a copper foil is placed and pressed at an upper end of the protruded conductor bump 2′ to form a circuit pattern with the copper foil. In this manner, an inter connector 1 is completed.
However, the above-described printed circuit board fabrication method has various disadvantages. The process for coating the conductive paste should be repeatedly performed to form the conductor bump 2′ with the predetermined height, which results in a complicated process. In addition, in order to increase the height of the conductor bump 2′, a diameter of a lower portion of the conductor bump 2′ should be above a certain size. Accordingly, the adjacent conductor bumps are distanced from each other, which makes it difficult to form a fine circuit pattern or obtain a compact printed circuit board. Moreover, in the repeated process of coating the conductive paste, an alignment process should be performed whenever the conductive paste is coated, which causes problems in that a productivity is degraded and a connecting force between the repeatedly coated conductive paste is also degraded.
FIGS. 2A and 2B show another conventional fabrication method of a printed circuit board disclosed in Japanese Laid Open Publication No. 2001-111189 (corresponding to U.S. Pat. No. 6,528,874) that will now be described. First, a base material 10 is prepared. As shown in FIG. 2A, the base material 10 is formed as a first copper layer 11 and a second copper layer 12 stacked together with an etching resist layer 13 therebetween.
The first copper layer 11 is for forming a connection protrusion 16 to electrically connect the stacked circuit patterns and is formed relatively thick. The second copper layer 12 is for forming the circuit pattern and is formed relatively thin.
A resist film 14 is formed at both upper surface and lower surface of the base material 10. The resist film 14 formed at the surface of the first copper layer 11 is removed to form a window 15 to form a connection protrusion 16. However, the resist film 14 is formed to cover entire surface of the second copper layer 12 as shown in FIG. 2B.
In this state, when an etching process is performed, an etching solution permeates the portion where the window 15 is formed to remove the copper layer 11 while leaving the copper layer 12 at the portion with the resist film 14 is coated. Thus, the connection protrusion 16 is formed as shown in FIG. 2C.
At this time, since etching is sequentially performed from the surface to the etching resist layer 13, the connection protrusion 16 is cone-shaped. A diameter of the connection protrusion 16 becomes smaller as it goes toward the end portion, which is called the etching factor. Since the second copper layer 12 is protected with the etching resist layer 13 and the resist film 14, the etching solution won't permeate it.
After the etching process is completed, the resist film 14 is removed. Then, the etching resist layer 13 is also removed by using an etching solution, which selectively etches only the etching resist layer 13 as shown in FIG. 2D.
Thereafter, a prepreg film 17 is placed over the connection protrusion 16 and pressed by using a roller. Then, the prepreg film 17 penetrates the connection protrusion 16 to be positioned between the connection protrusions 16 with the end portion of the connection protrusion 16 protruded on the surface of the prepreg film 17 as shown in FIG. 2E.
The end portion of the exposed connection protrusion 16 is abraded to be removed, and a third copper layer 18 is thermally pressed to form a circuit pattern. The second copper layer 12 and the third copper layer 18 are electrically connected by the connection protrusion 16 as shown in FIG. 2F. Thereafter, a circuit pattern 19 is formed through a general method on the second and third copper layers 12 and 18 to complete a printed circuit board as shown in FIG. 2G.
However, the printed circuit board fabrication method as shown in FIGS. 2A-2G has various disadvantages. As described above, to form the connection protrusion 16 with a certain height, the first copper layer 11 is formed thick, which causes much loss of a material. Further, in the etching process to form the connection protrusion 16, the diameter of the end portion of the connection protrusion 16 is smaller than the bottom portion because of the etching factor. Thus, in order to have a certain value of diameter of the end portion, the diameter of the bottom portion of the connection protrusion 16 needs to be increased. However, the diameter of the bottom portion sets an interval between the connection protrusions 16, which makes it difficult to narrow the gap between the connection protrusions 16 needed to form a fine circuit pattern. Further, the second copper layer 12 forming the circuit pattern has a thickness of below 20 mm while the first copper layer 11 forming the connection protrusion has a thickness of about 100 mm, which can cause the second copper layer 12 to be bent after the connection protrusion 16 is formed. Accordingly, it is difficult to handle the second copper layer 12.
In addition, the conventional printed circuit board fabrication methods described above use the screen printing method or the etching process to form the connection protrusion that can increase material loss, degrade rigidity of the connection protrusions and reduce productivity.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.